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            <a href="http://heatstress.com.my" title="Haba Stress Engineering">Haba <span>Stress</span> Engineering</a>
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<h2 id="faq-1">What Is Post Weld Heat Treatment</h2>
<p>One of the purposes of heat - treating is to relieve stresses caused when two pieces of metal are joint 
together by welding. As the metal in welded area raised to fusion temperature, the surrounding metal tends to 
expand but is restrained by the unheated areas adjacent to the welded joint. As a result, the heated portion is 
place under a compression stress, the magnitude of which may approach and sometimes exceed the yield point of 
metal.</p>

<p>Upon the cooling, the heated areas contract and set up reverse stresses, with the result that when cooled to 
room temperature the metal in the weld area is under constant Tensile Stress. To avoid internal stresses, 
stress-relieving equipment is equally applicable for normalizing, annealing and pre-heating, and electrical 
heating apparatus is proving successful in carrying out stress relieving requirements, for temperature up to 
1100&deg;C.</p>

<p>Control of temperature is an important feature and temperature in range of 600&deg;C to 950&deg;C required to 
heat-treat the materials can be arrived at a specified rate of temperature increase. The heat can then be 
adjusted to maintain the area being stress relieved at the temperature and period specified.
Reducing the temperature at any given rate controls the cooling period.</p>

<h2 id="faq-2">Question &amp; Answer</h2>
<p>Question: Why is post-weld heat treatment (stress relieving) sometimes necessary for welded vessels? </p>
<p>Answer:
        During the welding process, the two metal pieces being joined are subject to extreme temperatures and can 
        cause the crystalline structure of the metal to pass through various metallurgical phases. As a result, hardening
        (and embitterment) of the metal can occur to varying degrees (usually dependent on carbon content). 
        Heat treatment is designed to reduce the hardness in the heat affected zone of the metals and also increase 
        ductility in these sections. Various pressure vessel codes contain the specifics regarding the procedures for 
        post-weld heat treatment. Heat is usually held for one hour per inch of thickness of the metal. The temperature
        used is based on the "P-number" of the metals. P-numbers are assigned based on the chemical composition of the 
        metals. Holding temperatures can range from 1100-1350 &deg;F (593-732 &deg;C).</p>
        <p>Our personnel will supervise any stress relieving operations that the clients desires, making provision for yiel
        d at high temperatures and secondary stressing when cooling at the degree and duration of soaking temperature 
        necessary for the material</p>
        
        <h2 id="faq-3">Heat Treatment Terms</h2>
        <h3>Stress Relieving (Post Weld Heat Treatment)</h3>
        <p>Is the heating of steel to a relatively high temperature e.g. (590&deg; C -700&deg; C) shortly after welding, 
        and maintaining that temperature for a specific period of time. The purpose is to reduce internal stresses,
        caused by welding, quench hardening or by cold working. It also has an advantageous effect on the 
        elasticity, ductility and strength of the metal.</p>
        
       <h3>Pre - Heating</h3>
       <p>Is a very effective means of preventing weld metal or base metal from cracking? It is the application 
       of heat to the base metal prior to a welding or cutting operation. It does not change the properties of 
       the steel, but it is advisable under certain conditions to drive off moisture and help to ensure even 
       expansion to eliminate undesirable stresses. Concurrent heating during welding (also referred to as 
       pre - heating) retards the cooling thus preventing the formulation of an under bead crack.</p>
       
       <h3>Solution Heat Treatment</h3>
       <p>Is performed on certain types of authentic stainless steel to prevent decay in WELDMENT. 
       This is done by the high temperatures (e.g. 1050&deg;C) and the cooling is very rapid.</p>
       
       <h3>Normalizing</h3>
       <p>Is much like annealing, but the cooling process is much faster. This result is in increased strength 
       but less ductility in the metal. It also produces higher yield strength with greater tensile strength, and
       in most instances, a higher impact resistant.</p>
       
       <h3>Tempering</h3>
       <p>Is employed after steels have been fully hardened and are too brittle to be of any practical use. 
       Tempering reduces the brittleness and relieves internal stresses. Tempering is actually a heating and slow
       cooling process. The cooling is accurately controlled to impact certain characteristics to the base metal.</p>
        
       <h3>Annealing</h3>
       <p>Was the first form of heat treatment applied to the ferrous metals. It is essentially the softening of
       the base metal to make it more malleable. The process is involves a controlled heating and slow cooling 
       which includes softness. Additionally, the process removes gasses, relieves stresses and enhances the 
       mechanical properties in alloyed steels. This is generally performed at temperatures up to 700&deg;C</p>
       
       <h3>Quench Hardening</h3>
       <p>Is process of cooling heated metal to room temperature by immersion in liquid or gaseous cooling medium.
       This method provides greater strength and hardness than slow cooling, but it is accompanied by brittleness.</p>
       
       <p><a href="#nav">Scrol to Top</a></p>
       
       <h2 id="faq-4">Definitions</h2>
      <h3>Post-weld heat treatment</h3>
      <p>Heat treatment carried out after welding in order to decrease residual welding stress and/or change the 
      micro structure.</p>
      
      <h3>Post-forming heat treatment</h3>
      <p>Heat treatment carried out after forming in order to regain the original properties of the material or 
      reduce stresses caused by the forming.</p>
      
      <h3>Holding temperature</h3>
      <p>The holding temperature is the temperature range in which the product or component is kept in order to 
      achieve specified properties. The holding temperature depends on the type of heat treatment and material. 
      Normally it is expressed as a temperature range (580&deg;C - 620 &deg;C).</p>
      
      <h3>Holding time</h3>
      <p>Holding time is the time the product or component is kept at the holding temperature. The holding time
       starts when the temperature at all the measuring points has reached minimum value of the range of the 
       holding temperature and stops when one of the measuring points falls below that temperature. The duration 
       of the holding time depends on the type of heat treatment, material and material thickness.</p>
       
       <h3>Unloading temperature</h3>
       <p>The temperature of the product or component when it is taken out e.g. of a furnace or when any other 
       heat treatment is finished.</p>
       
       <p><a href="#nav">Scrol to Top</a></p>
       
      <h2 id="faq-5">Objectives of Heat Treatments</h2>
      <p><strong>Heat Treatment</strong> is the <em>controlled heating and cooling of metals to alter their physical and mechanical 
      properties without changing the product shape</em>. Heat treatment is sometimes done inadvertently due to 
      manufacturing processes that either heat or cool the metal such as welding or forming.</p>
      <p>Heat Treatment is often associated with increasing the strength of material, but it can also be used to 
      alter certain manufacturability objectives such as improve machining, improve formability, restore 
      ductility after a cold working operation. Thus it is a very enabling manufacturing process that can not 
      only help other manufacturing process, but can also improve product performance by increasing strength or 
      other desirable characteristics.</p>
      <p>Steels are particularly suitable for heat treatment, since they respond well to heat treatment and the
      commercial use of steels exceeds that of any other material. Steels are heat treated for one of the 
      following reasons:
      <ul>      
        <li>Softening</li>
        <li>Hardening</li> 
        <li>Material Modification</li>
        </ul>
        </p>
        
        <p><a href="#nav">Scrol to Top</a></p>
        
    <h2 id="faq-6">Common Heat Treatments</h2>
    <p><strong>Softening</strong>: Softening is done <em>to reduce strength or hardness, remove residual stresses, improve toughnesss, 
    restore ductility, refine grain size or change the electromagnetic properties of the steel.</em></p>
    
    <p>Restoring ductility or removing residual stresses is a necessary operation when a large amount of cold 
    working is to be performed, such as in a cold-rolling operation or wiredrawing. Annealing  full Process, 
    spheroidizing, normalizing and tempering - austempering, martempering are the principal ways by which steel 
    is softened.</p>
    
    <p><strong>Hardening</strong>: Hardening of steels is done to increase the strength and wear properties. One of the 
    pre-requisites for hardening is sufficient carbon and alloy content. If there is sufficient Carbon content 
    then the steel can be directly hardened. Otherwise the surface of the part has to be Carbon enriched using 
    some diffusion treatment hardening techniques</p>
    
    <p><strong>Material Modification</strong>: Heat treatment is used to modify properties of materials in addition to hardening 
    and softening. These processes modify the behavior of the steels in a beneficial manner to maximize service 
    life, e.g., stress relieving, or strength properties, e.g., cryogenic treatment, or some other desirable 
    properties, e.g., spring aging.</p>
    
    <p><a href="#nav">Scrol to Top</a></p>
    
    <h2 id="faq-7">Why Use a Controlled Preheat?</h2>
    <h3>Preheating During Welding provides the following benefits:</h3>
    <ul class="wrap">
    <li>Reduces the levels of thermal stress.</li>
    <li>Provides compensation for high heat losses.</li>
    <li>Minimizes the rate of weld hardening.</li>
    <li>Reduced porosity in the weld.</li>
    <li>Reduced hydrogen cracking.</li>
    <li>Improved microstructure of heat affected zone.</li>
    </ul>
    
    <p><a href="#nav">Scrol to Top</a></p>
    
    <h2 id="faq-8">Vertical heat treatment</h2>
    <p>The suction roll shell undergoes heat treatment in a vertical position in the furnace to ensure that its 
    round shape is not distorted by its own weight. The entire heat treatment process can be divided into three 
    stages: heating, holding time and cooling. The cooling stage is crucial for obtaining the right properties. 
    A uniform cooling rate is achieved by blowing air along the inside of the suction roll shell during the 
    cooling process. The difference in temperature between the inside and outside of the suction roll shell must 
    never exceed 150&deg;C (302&deg;F). In practice, it is no more than 100oC (2 12&deg;F). The entire process of heating, 
    holding time and cooling is controlled by thermal couples placed around the inner and outer diameter of the 
    suction roll shell along its entire length.</p>
    
      <p><a href="#nav">Scrol to Top</a></p>
      
      <h2 id="faq-9">Compressive stress on the inner and outer walls</h2>
      <p>The controlled heat treatment process improves the service life of the suction roll shell. By 
      controlling the cooling process so that the temperature on both the inside and outside of the suction roll 
      shell is constantly lower than that at its core, a compressive stress is created on the inside and outside 
      of the shell. Compressive stress means that the suction roll shell can handle greater loads and is more 
      resistant to corrosion fatigue in those parts where cracking is otherwise most likely to occur.</p>
       
        <p><a href="#nav">Scrol to Top</a></p>
    </article>
     
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    <h3 class="title">Sitemaps</h3>
        <ul>
            <li><a href="#faq-1">What is Post Weld Heat Treatment?</a></li>
            <li><a href="#faq-2">Question &amp; Answer</a></li>
            <li><a href="#faq-3">Heat Treatment Terms</a></li>
            <li><a href="#faq-4">Definitions</a></li>
            <li><a href="#faq-5">Objectives Of Heat Treatments</a></li>
            <li><a href="#faq-6">Common Heat Treatments</a></li>
            <li><a href="#faq-7">Why Use A Controlled Preheat?</a></li>
            <li><a href="#faq-8">Vertical heat treatmen</a></li>
            <li><a href="#faq-8">Compressive stress on the inner and outer walls</a></li>
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